Process And Properties Of SiC_f/SiC Joints Brazed By Ca-Mg-Al-Si-O And Y-Al-Si-O Glass

In the Fukushima nuclear accident in Japan,the Zr alloy cladding reacted with water vapor to generate a large amount of hydrogen gas,causing an explosion and causing a serious accident.The development of a new generation of accident-tolerant nuclear fuel cladding tubes has become a research hotspot.SiC_f/SiC has low neutron absorption cross section,radiation resistance,high temperature resistance,high thermal conductivity,high specific strength,high specific stiffness,and thermal shock resistance.So,it becomes the ideal nuclear fuel cladding material.At present,the preparation and port connection/packaging technology of SiC_f/SiC cladding tube is a key problem that restricts its application to pressurized water reactors.This paper mainly studies the nuclear fuel cladding port connection/packaging technology of SiC_f/SiC composites.CaO-MgO-Al₂O₃-SiO₂?CMAS?and Y₂O₃-Al₂O₃-SiO₂?YAS?glasses have a small neutron absorption cross section,and the thermal expansion coefficients are matched with SiC_f/SiC composites.The connection process can be carried out under lower temperature and no pressure/low pressure conditions,and is suitable for the connection of the domestic SiC_f/SiC composite core cladding.In this paper,the connection of SiC_f/SiC composites was realized by CMAS and YAS glass.The influence of the joining process on the microstructure and properties of the joints in different glass systems was studied.The joint process was optimized and the optimum process connection specimens were evaluated.The thermal shock resistance is achieved,and the port package of the SiC_f/SiC cladding tube is completed by CMAS and YAS glass.The main findings are as follows:1.The connection was explored using MgO-Al₂O₃-SiO₂?MAS?and CaO-Al₂O₃-SiO₂?CAS?glass.The results show that the MAS glass forms cordierite after heat treatment,which leads to the decrease of glass phase content and viscosity.It is difficult to connect SiC_f/SiC composites under 1400°C pressureless conditions.The thermal expansion coefficient of CAS glass is large.Thermal mismatch produces significant cracks with low joint strength and does not guarantee joint tightness.2.The effects of CMAS glass ratio,joint temperature and holding time on the microstructure and shear strength of SiC_f/SiC joints were studied.The joint process was optimized,and the thermal shock resistance of joints was evaluated.The results show that the glass joints with the ratio of?SiO₂ 64.2wt.%,Al₂O₃ 17.8wt.%,CaO 9.0wt.%,MgO 9.0wt.%?are dense and defect-free and have the highest strength;As the connection temperature increases,the shear strength of the joint increases,and the degree of internal crystallization increases,and some defects exist in the joint layer;When the connection temperature is raised to 1400°C,the connecting layer is a uniform glass phase and the structure is the most dense;At 1400°C,the holding time has little effect on the microstructure and strength of the connected samples;The optimized joining process is 1400°C/60 mins,and the joint shear strength is⁴5.7 MPa.The shear strength of the sample decreases with the increase of the number of thermal shocks.After three thermal shocks,the shear strength is²6.4 MPa.3.The effects of YAS glass ratio,joint temperature and holding time on the microstructure and shear strength of SiC_f/SiC joints were studied and revealed.The connection process was optimized,and the thermal shock resistance of the joint was evaluated.The results show that the glass joints with the ratio of?SiO₂ 55.0wt.%,Al₂O₃15.0wt.%,Y₂O₃ 30.0wt.%?are dense and the internal crystal distribution is uniform;As the connection temperature increases,a continuous dense structure gradually forms in the joint layer,and the shear strength first rises and then stabilizes;Under the optimal connection temperature condition,with the increase of holding time,the connecting layer in the sample is gradually dense,and the shear strength first rises and then stabilizes;The optimized joining process is 1400°C/30 mins and the shear strength of the sample is⁴3.7 MPa.The shear strength of the sample decreases with the increase of the number of thermal shocks.After three thermal shocks,the shear strength of the sample is¹0.8 MPa.4.The SiC_f/SiC core cladding tube was encapsulated by CMAS and YAS glass under its optimal process.The results show that both glasses can enclose the SiC_f/SiC core cladding tube,and the package area is dense.At the same time,the glass solder penetrated into the pores in the SiC_f/SiC plug,which healed some defects in the SiC_f/SiC.This helps to ensure the airtightness at the port of the cladding tube.There is a small amount of closed cells in the package area.